Generally, a piston P that reciprocates in a cylinder bore of an internal combustion engine has three ring grooves Pg as shown in FIG. 1. Two compression rings 1 and 2 used to prevent the blow-by of a combustion gas and an oil ring 3 used to adjust a lubricant film are fitted in these ring grooves Pg. One of the two compression rings is a first ring 1 that chiefly seals the combustion gas, and the other is a second ring 2 that seals the combustion gas and adjusts the lubricant film. A connecting rod R is swingably connected to the piston P.
Generally, a tapered-face type ring whose outer peripheral surface is inclined is employed as the second ring 2. For example, if an end face 2a thereof directed toward an upper surface Pu of the piston P is defined as a front face, and an end face 2b thereof directed toward a lower surface of the piston P is defined as a back face in a state of being correctly fitted around the piston P, the outer peripheral surface 2c of the second ring is formed to have a tapered shape (a truncated-cone shape) in which the outer diameter of an edge on the front face side becomes smaller, whereas the outer diameter of an edge on the back face side becomes greater.
Therefore, the second ring 2 is required to be correctly fitted into the ring groove Pg of the piston P so as not to allow the inclination direction of the outer peripheral surface 2c to become opposite, i.e., so as not to incorrectly set the front and the back of the second ring 2.
Therefore, in order to inspect whether or not this piston ring (the second ring 2) has been fitted around the piston P in a correct direction, a conventional technique has been carried out such that a laser sensor that measures a distance by use of spot-like detection light is allowed to scan the outer peripheral surface of the piston P, and measurement data obtained by the laser sensor and positional data of the laser sensor are analyzed and processed by dedicated software so as to calculate the shape of the outer peripheral surface of the second ring 2, and, based on the calculation result, the front and the back of the second ring are judged.
However, in the conventional technique, the measurement data of the laser sensor and the positional data of the laser sensor must be analyzed while being synchronized, and a device, such as an encoder, to obtain positional data is needed. Additionally, complex analysis software, or the like, used to calculate the shape of the ring from the measurement data and the positional data is needed. Still additionally, in order to scan the outer peripheral surface of the piston P, the laser sensor or the piston is required to be relatively moved, and hence there is a fear that an error will occur in the measurement data because of vibrations or shakes caused by the movement of the laser sensor or the piston.
A known conventional laser sensor measures the outline and the like of an object by using slit light (detection light) whose cross-section is linear (see Japanese Unexamined Patent Publication No. 2002-357408, for example). However, the laser sensor disclosed in this publication can measure the outline of the object, but cannot automatically judge whether it is OK or NG based on, for example, its measurement data. Additionally, in this laser sensor, a measuring range cannot be easily and freely selected based on, for example, an obtained image, and hence wasteful time is consumed in measurement.
The present invention has been made in consideration of the foregoing circumstances of the conventional technique. It is therefore an object of the present invention to provide a piston-ring inspecting device and method capable of measuring and approximating the shape of the outer peripheral surface of a piston ring in a short time and with high accuracy through a simple analysis process without needing a relative movement (scanning) of a piston and a sensor and capable of judging whether or not the piston ring has been fitted around the piston in a correct direction.